1. Field of the Invention
This invention relates to a portable power system using fuel cells, and more particularly to a fuel cell apparatus employing polymer electrolyte fuel cells using the air as an oxidizer.
2. Related Art
Examples of prior art techniques, using a fuel cell as a portable power source, are disclosed in JP-A-04-308662 and JP-A-06-60894, and these publications disclose a construction in which a phosphoric acid fuel cell is operated by hydrogen, supplied from a hydrogen storage alloy, and the air. JP-A-54-22537 and JP-A-02-260371 disclose a construction in which a polymer electrolyte fuel cell is operated by hydrogen, supplied from a hydrogen storage alloy, and the air.
In a polymer electrolyte fuel cell, a proton exchange membrane (PEM), which is a polymer electrolyte, is used as an electrolyte, and its general construction is shown in FIG. 3. In the construction using this proton exchange membrane 17, a layer of a positive electrode (oxygen electrode) 18 and a layer of a negative electrode (hydrogen electrode) 19 are formed respectively on opposite sides of the proton exchange membrane 17, and these jointly constitute a unit cell 20.
In the case where hydrogen is used as a fuel while oxygen is used as an oxidizer, a reaction, expressed by the following formula (1), occurs at the negative electrode at the interface of contact between a catalyst and the polymer electrolyte while a reaction, expressed by the following formula (2), occurs at the positive electrode, so that water is formed. EQU H.sub.2 2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (2)
The catalyst serves to provide an active site or spot of the reaction, and the active sites serve as a conductor for the electrons in the above reactions, and the polymer electrolyte serves as a conductor for the hydrogen ions. However, the polymer electrolyte does not exhibit ion-permeability before it becomes moistened, and therefore with respect to a feature of the power system employing the polymer electrolyte fuel cell, a method of moistening the polymer electrolyte has been extensively studied. The unit cells 20 are connected in series by using separator plates 21 and gaskets 22 (see FIG. 4) to form a laminate 23 (see FIG. 5) which is fastened by end plates 24 to thereby provide one electricity-generating unit.
During the generation of electricity, the energy of an excess voltage, corresponding to a current density at which the electricity is generated, is discharged from the fuel cell body of this construction, and therefore the fuel cell body serves as a heat-generating source.
The hydrogen storage alloy of the hydrogen storage tank for supplying hydrogen to the fuel cell performs a representative reaction, expressed by the following formula (3), in accordance with the storage and discharge of hydrogen: ##STR1##
M: hydrogen storage alloy, H: hydrogen .alpha., .beta.: ratio of hydrogen atoms H to hydrogen storage alloy atoms M in a solid phase (This ratio corresponds to a stoichiometric composition of a hydride phase)
The hydrogen content of the metal, exhibiting a phase (which is a metal phase in which hydrogen is dissolved), increases, and at the time of the reaction (the reaction in a right-hand direction in formula (3)) when the .alpha. phase reacts with hydrogen gas and is converted into .beta. phase (hydride), which is a hydride phase, heat .DELTA.H of formation is produced. When hydrogen is emitted from the metal hydride, the .beta. phase is converted into the .alpha. phase, thereby absorbing the heat .DELTA.H, and this characteristic is already known. At this time, in order to stably. supply hydrogen, it is necessary to supply heat to the hydrogen storage alloy, and therefore there have been proposed various methods of supplying heat to the hydrogen storage tank.
However, in the above conventional portable fuel cell and the above conventional polymer electrolyte fuel cell system, any consideration has not been given to a construction for achieving a compact design in view of the heat transmission between the fuel cell body, serving as the heat-generating source, and the hydrogen storage alloy portion serving as the heat-absorbing source.
For example, in the construction disclosed in JP-A-54-22537 and JP-A-02-260371, the polymer electrolyte fuel cell is operated by hydrogen supplied from the hydrogen storage alloy, but these publications show only the construction for transmitting heat of the fuel cell to the hydrogen storage alloy, the construction of a wick member for recovering the formed water, and the construction of a water-permeable member, and do not disclose any construction for achieving the compact design. U. S. Pat. No. 5,200,278 discloses various techniques related to the construction of a polymer electrolyte fuel cell and such a fuel cell system, but does not suggest any construction for achieving a compact design, and there is a problem that any consideration has not been given to a construction for achieving the compact design in view of the heat transmission between the fuel cell body, serving as the heat-generating source, and the heat of the hydrogen storage alloy portion serving as the heat-absorbing source.